In the manufacture of metal tubing components for hydraulic or pneumatic circuits on a mass-production basis, as, for example, automotive applications, it is common practice to employ a screw machine to cut uniform lengths in succession from one end of a relatively long piece of tubing stock.
In this operation, a length of tubing stock, typically 12 feet in length, is advanced axially centrally through the rotary chuck of a screw machine until the leading end of the stock projects a predetermined distance forwardly from the chuck. The chuck is then closed to drive the stock in rotation while a forming operation is performed on the projecting length of tubing. The forming operation may consist simply of a cutting-off operation in which a cutting tool is advanced radially to the rotating tube to cut off a predetermined length from the projecting end of the tube, usually chamfering the end as it is so cut. The cut-off part is then ejected, the chuck is opened and the tubing stock is advanced through the chuck to repeat the process.
As stated above, the typical length of tubing stock employed in the foregoing operation is 12 feet and, while the forming or severing operation is being performed on the leading end of the tubing, the entire length of tubing is driven in rotation by the rotary chuck. Because the chucks in question are driven in rotation at speeds of 3,000 to 4,000 RPM, it is essential that the tubing stock handled by the machine be accurately axially straight. Stock suitable for this type of operation is referred to as "rotary straight" tubing stock. If the stock is not "rotary straight", the relatively high speed of rotation will result in unacceptably high vibratory forces. "Rotary straight" tubing stock is normally required to deviate from a straight line by less than 0.002 inches per foot of length.
Because tubing employed in hydraulic and pneumatic circuitry frequently must be bent into various unusual configurations, the tubing material conventionally is a relatively soft metal or alloy which may be easily bent and will hold its bent shape. The property enables the tubing to be wound into multilayer helical coils upon reels for convenient storage and shipment, and most tubing manufacturers arrange their production facilitates so that the tubing is so coiled as produced. While tubing manufacturers will supply tubing in rotary straight lengths, tubing supplied in rotary straight lengths typically commands a price premium of 75% to 100% per unit length over the same tubing supplied in coils.
The coiled tubing will take a so-called "coil set" which can be corrected without undue inconvenience in many applications, particularly those involving low-volume production rates. However, for high-volume production in which parts are processed in automatic screw machines, the problems encountered in straightening coiled tubing to rotary straight tolerances adequate for usage in automatic screw machines have been such that the tubing component manufacturer must choose between the lesser of two evils. Either he must forego the production efficiency of an automatic screw machine or he must pay a premium price to the manufacturer for rotary straight tubing.
For a discussion of the problems encountered in straightening coiled wire, see Labbe U.S. Pat. No. 4,464,919 which also refers to several prior United States patents directed to apparatus for straightening wire. As explained in more detail in U.S. Pat. No. 4,464,919, a primary problem is presented by the fact that the straightening device, which normally consists of a series of rollers, is mounted upon a fixed frame and the supply coil is mounted for rotation upon a fixed stand. The angle at which the wire moves into the straightening device continuously changes as the wire is uncoiled from the reel. This not only results in undue wear of the first group of rollers through which the wire passes, but also results in an uneven or inconsistent straightening of the wire.
The present invention is especially directed to a method and apparatus for producing rotary straight lengths of tubing stock from coiled tubing.